In recent years, for glass products to be used in the field of optical communication or in the semiconductor industry, a very strict control has been carried out with respect to minor impurities and fine bubbles in the products. Such high quality glass is produced mainly by e.g. (1) a method for purifying natural quartz, (2) a method wherein a fume formed by decomposition of silicon tetrachloride in an oxyhydrogen flame is deposited and grown on a substrate, or (3) a method wherein a silica gel obtained by hydrolysis and gelation of a silicon alkoxide or the like, is baked to obtain a synthetic quartz powder, and such a quartz powder is fused to obtain a shaped product.
However, the method (1) has a problem that there is a limit in reduction of the content of minor impurities, and the method (2) has a problem that the production cost is extremely high. On the other hand, the method (3) of baking a silica gel does not necessarily satisfy the required level, although as compared with the method (2), it is possible to obtain a synthetic quartz powder having a low content of minor impurities at a low cost. Besides, this method has a problem that fine bubbles may sometimes form in the shaped product as the final product, and such fine bubbles tend to cause various troubles.
The present inventors have conducted extensive studies to find out a solution to the problem in the above method (3) for producing a synthetic quartz powder by baking a silica gel, i.e. to find out a method for producing a synthetic quartz powder whereby formation of fine bubbles in a shaped product obtainable by fusing it is very little, and a method for carrying out such production industrially advantageously, and the following points have been made clear. Namely, baking of a silica gel is carried out by charging a silica gel into a quartz container in order to avoid contamination of impurities from the container and heating it in e.g. an electric furnace. Especially for industrial production, a quartz crucible having a large diameter or the like is used. However, a silica gel has a low bulk density as compared with the quartz powder, and accordingly, the container used for baking can not effectively be utilized, whereby the productivity is poor, and the production cost is high. Accordingly, to improve the productivity, it is important to increase the bulk density of the powder to be charged into the crucible.
On the other hand, in the production of a shaped product employing a quartz powder, formation of fine bubbles during the production of the shaped product, is influenced by the temperature-raising process in the baking step for the production of the quartz powder. In the silica gel powder obtained by hydrolysis of a silicon alkoxide, unreacted alkoxy groups and a part of an alcohol formed by a side reaction will remain even if the alcohol formed by a side reaction has been removed by drying. In fact, when the carbon concentration in the silica gel powder dried, is measured, it is from 1 to 3%, although it varies depending upon the drying condition. If this silica gel powder is baked in an oxygen-containing gas, the majority of carbon can be removed by combustion during the temperature-raising process. However, a part thereof may sometimes be included in the synthetic quartz powder as non-combustion carbon. If this synthetic quartz powder containing such non-combustion carbon, is used, CO or CO.sub.2 gas will be formed during melt-forming, which causes formation of bubbles. Accordingly, it is necessary to remove substantially the entire amount of non-combustion carbon before the pores of the silica gel be closed, and the temperature-raising speed in the temperature-raising process becomes important.
However, as mentioned above, when industrial production of a synthetic quartz powder is intended, a quartz crucible having a large diameter is required, and the temperature in the crucible during the temperature-raising process tends to be non-uniform, and it is difficult to raise the temperature over the entire portion in the crucible at a predetermined temperature pattern. As a result, in some cases, a synthetic quartz powder containing residual carbon is likely to partially form, and fine bubbles are likely to form in a shaped product prepared by using such a synthetic quartz powder.
The present inventors have tried heating by means of a rotary cylindrical heating apparatus, a so-called rotary kiln, in order to form a synthetic quartz powder of good quality efficiently by solving such a problem. However, for the purpose of avoiding contamination, the material of the core barrel is limited to quartz or the like, whereby there has been a problem that the useful life of the core barrel tends to be short due to the heat shock by repetition of the temperature-raising step in the heat treatment. Further, a large amount of gas is generated from the silica gel by the heat treatment, and the amount of the gas generated, changes with time, whereby the waste gas treatment is difficult by the rotary kiln, and to meet with the peak in the generation of gas, the apparatus is obliged to be of a large size, and the generated gas may sometimes blow off the inner silica gel powder.
Further, in order to adequately and precisely carry out removal of the solvent, attached water, residual organic groups, etc. in the step of gradually closing the pores of the silica gel to obtain the desired synthetic quartz powder of high quality, it is necessary to strictly control the conditions for heat treatment, especially the temperature-raising step, and it is difficult to carry out such a strict control by a rotary kiln charged with a large amount of a powder. Further, there has been a problem that variation in the heating intensity is likely to occur in the axial direction of the rotary kiln, which leads to non-uniformity of the product.
Further, prevention of contamination during supply and withdrawal of the powder and unifying the quality of every batch, have not adequately be accomplished.